High frequency optical processing

ABSTRACT

This invention concerns a high frequency optical processor suitable for time-division demultiplexing and channel dropping data signals at bit rates of 80-160 Gb/s and beyond, and for carving high frequency pulse streams with low duty cycle from an unmodulated laser source. In further aspects the invention concerns methods for using the high frequency optical processor. In particular the invention comprises a high frequency optical processor, comprising a Mach-Zehnder amplitude modulator (MZM) embedded in a fibre mirror loop in which the loop input and output are joined by a polarization maintaining coupler.

TECHNICAL FIELD

This invention concerns a high frequency optical processor suitable for time-division demultiplexing and channel dropping data signals at bit rates of 80-160 Gb/s and beyond, and for carving high frequency pulse streams with low duty cycle from an unmodulated laser source. In further aspects the invention concerns methods for using the high frequency optical processor.

BACKGROUND ART

Direct data modulation and detection of optical data signals at bit rates of 80-160 Gb/s and beyond is constrained by the modulation bandwidth of electro-optic devices, which are conventionally less than 40 GHz. A possible solution is optical time-division multiplexing (OTDM) whereby multiple 40 Gb/s channels of the same wavelength are temporally interleaved to form the higher bit rate signal [1].

At the receiver, electronic detection and bit error rate measurement are enabled by demultiplexing the OTDM signal to its constituent sub-rate channels. Typically, this can be done by fast optical gating to extract each sub-rate channel pulse using either an electro-optic device such as an electro-absorption modulator [2-4], or all-optical schemes involving interaction in nonlinear medium with co-propagating synchronized pump pulse [5-7]. While the electro-absorption modulator is simple to implement, demultiplexing performance for standard devices is inferior at bit rates of 160 Gb/s due to inadequate switching time.

Recently, a new scheme for high performance demultiplexing a 160 Gb/s signal to 40 Gb/s (i.e. 4:1 demux) was demonstrated using an a LiNbO₃ phase modulator in conjunction with post-optical filtering [8]. It was shown that 40 GHz modulation of the 160 Gb/s signal manipulates the phase change over consecutive pulses in such a way to allow a 40 Gb/s channel to be discriminated by optical filtering. By similar principle, 2:1 demultplexing of 100 Gb/s signal has been shown using a phase modulator connected in a fiber loop [9], however in contrast to the filtering technique, this method requires cascading two modulators driven at different harmonic frequencies to enable equivalent 4:1 demultiplexing. Directly cascading LiNbO₃ amplitude modulators for demultiplexing has also been reported whereby the delay between modulators is adjusted so that the combined switching response produces a shorter gating window [10].

DISCLOSURE OF THE INVENTION

This invention is a high frequency optical processor, comprising a Mach-Zehnder amplitude modulator (MZM) embedded in a fibre mirror loop (together FL-MZM) in which the loop input and output are joined by a polarization maintaining coupler.

As a result of the configuration, the counterpropagating waves within the loop experience unequal amplitude modulation leading to output transmission of a high quality demultiplexed (DEMUXED) pulse train.

The processor is suitable for time-division demultiplexing data signals at bit rates of 80-160 Gb/s, and beyond, input to the loop. Bit-error rate measurements show high-performance 4:1 demultiplexing of a 160 Gb/s optical signal to 40 Gb/s channels.

Also, the nature of the fiber loop operation enables simultaneous channel add/drop functionality with recovery of the remaining tributary channels for further processing, all with a single device in contrast to other reported schemes.

Increasing the modulation frequency to will enable scaling of the technique to provide a robust, high performance extraction of higher frequency channels with add/drop functionality.

The processor is also suitable for carving high frequency pulse streams of low duty cycle from a continuous wave (unmodulated) laser applied at the input to the loop.

The MZM may have a modulation port to receive a modulating voltage, and a bias port to receive a bias voltage.

In further aspects the invention concerns methods for using the high frequency optical processor.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the invention will now be described with reference to the accompanying drawings, in which:

FIG. 1( a) is a schematic diagram of a Mach-Zehnder amplitude modulator (MZM) embedded in a fibre loop (FL-MZM) according to the invention.

FIG. 1( b) is a graph showing the calculated waveforms for A₁₂ and A₂₁ for input A₀ of constant amplitude when V_(DC)=0.79 V_(π) and V_(P)=0.5 V_(π) at modulation frequency, R.

FIG. 1( c) is a graph showing the calculated waveform for A₃ for input A₀ of constant amplitude when V_(DC)=0.79 V_(π) and V_(P)=0.5 V_(π) at modulation frequency, R.

FIG. 1( d) is a graph showing the numerical drive voltage V for V_(P)=0.5 V_(π) and V_(DC)=0.79 V_(π).

FIG. 1( e) is a graph showing the corresponding fiber-loop transmitted output, and (dotted) the Gaussian fit.

FIG. 1( f) is the numerical (solid curves) MZM fiber-loop reflected output for (dots) 160 GHz pulsed input compared to (dashes) output for CW optical input, in the case of R=40 GHz, V_(P)=0.97 V_(π), and V_(DC)−0.5 V_(π).

FIG. 1( g) is the numerical (solid curves) MZM fiber-loop transmitted output for (dots) 160 GHz pulsed input compared to (dashes) output for CW optical input, in the case of R=40 GHz, V_(P)=0.97 V_(π), and V_(DC)−0.5 V_(π).

FIG. 2 is a schematic diagram showing the experimental set-up for demultiplexing (DEMUX) of a 160 Gb/s optical signal.

FIG. 3( a) is a graph showing the measured 40 GHz output pulse autocorrelation for CW laser input at 1550 nm wavelength to 40 GHz modulated FL-MZM with V_(P)˜V_(π).

FIG. 3( b) is a signal eye diagram at 1550 nm wavelength of a 40 Gb/s “back to back” (B2B) before multiplexing (MUX).

FIG. 3( c) is a signal eye diagram at 1550 nm wavelength of the 160 Gb/s after mulitplexing (MUX) of the signal of FIG. 3( b).

FIG. 3( d) is a signal eye diagram at 1550 nm wavelength of the 40 Gb/s after demultiplexing (DEMUX) of the signal of FIG. 3( c) using a 40 GHz modulated FL-MZM.

FIG. 4 is a graph showing BER performance for 160 GHz DEMUX of a 40 GB/s channel by a 40 GB/s modulated FL-MZM.

BEST MODES OF THE INVENTION

Referring first to FIG. 1 the Mach-Zehnder amplitude modulator (MZM) embedded in a fibre loop (FL-MZM) [14] and its particular arrangement will now be described: The Mach-Zehnder amplitude modulator (MZM) 10 comprises input port 12 connected via optical fibre 14 to an optical input 16, and an output port 18 connected via optical fibre 20 to an optical output 22. The two lengths of optical fibre 14 and 20 are formed into a loop mirror 24 using a polarization maintaining (PM) coupler 26 with coupling ratio, ρ. An input signal of amplitude A_(o) is split into two counter-propagating fields that circulate the fiber-loop in opposite directions and return to the coupler with different amplitudes (assuming co-polarized fields) of:

in the forward direction

A ₁₂=(T ₁₂·ρ)^(1/2) A ₀

and in the reverse direction

A ₂₁ =i(T ₂₁[1−ρ])^(1/2) A ₀

where T₁₂ and T₂₁ represent the MZM bi-directional optical power transmission functions for propagation from input port 12 to output port 18 and vice versa. Note the coupler induced π/2 phase shift for A₂₁.

The optical transmission of a MZM is a raised cosine function [13]. As a result in the forward direction, T₁₂ is given by:

T(V)=α·sin²(π·V/2V _(π)),

where V_(π) is the driving voltage for switching T from zero to its maximum, αthe insertion loss factor (which is ignored by assuming α=1).

V is the applied voltage defined as V(t)=V_(P)·cos (2π·R·t)+V_(DC), where V_(DC), is the DC bias, and V_(P) and R are the zero-to-peak amplitude and frequency of modulation respectively.

Since a high-speed MZM typically has a travelling wave design where velocity matches the applied voltage and optical fields in one direction, the effective V_(P) for counter-propagating optical field (input to 18) is small [15] causing T₁₂ to be different from T₂₁. Measurements for the MZM confirm negligible backward modulation, i.e. T₂₁˜T(V_(DC)).

The coupler transmitted output A₃ is given by the interference of A₁₂ and A₂₁ as:

A ₃=(ρ)^(1/2) A ₁₂ +i(1−ρ)^(1/2) A ₂₁=[ρ(T ₁₂)^(1/2)−(1−ρ)(T ₂₁)^(1/2) ]A ₀.   (1)

Pulse Carving

It can be seen that V_(DC) could be adjusted so that T₁₂ for (V−V_(DC))<0 confines A₁₂ around the peak of T with an average amplitude about equal to A₂₁. Also (V−V_(DC))<0, causes A₁₂ to form an inverted pulse, as a result a low duty cycle short pulse results for A₃.

In the case of constant A₀ input, V_(P)=0.5 V_(π), R=40 GHz, ρ=0.5 and an optimum V_(DC) of 0.79 V_(π). As shown in FIG. 1( b), |A₁₂|² forms an inverted pulse (of repetition, R) on a background of average ˜|A₂₁|². By equation (1) for A₃, the background is cancelled by A₂₁ and a nearly pedestal free pulse is emitted, as shown in FIG. 1( c).

The output pulse has a full width at half maximum (FWHM) of 5.4 ps and a peak power 19.7 dB above the peak pedestal; which is more favourable than the Gaussian (shown dotted). The throughput loss in peak power, G, is 12 dB.

Similar A₃ is obtained for other combinations of V_(P) and V_(DC); a higher Vp=V_(π) with V_(DC)=0.59 V_(π) gives similar FWHM of 6.2 ps, with advantageously higher G of 3.1 dB.

Demultiplexing (DEMUX)

The principle of using the device for demultiplexing operation is the same as described above for pulse carving with the same modulation and biasing conditions for the MZM embedded in a fiber loop. The only difference is that the optical input is a pulsed signal instead of an unmodulated laser source.

In this example, referring to the setup of FIG. 2 and the graphs of FIG. 3, optical time division multiplexing (OTDM) is used to increase the data transmission rate per wavelength beyond the electro-optic bandwidth limits of optical signal transmitters and receivers. The system interleaves four 40 Gb/s channels modulated on the same wavelength to reach 160 Gb/s bit-rate [11], [12] and these are then going to be demulitplexed.

The 160 Gb/s return-to-zero (RZ) signal was generated from an active mode-locked fiber laser (MLFL) 40 emitting 40 GHz repetition-rate pulses of 2.4 nm bandwidth centered at 1550 nm wavelength. An external MZM 42 encoded data on the pulses at 40 Gb/s with a 231-1 pseudo-random bit pattern producing the signal eye diagram shown in FIG. 3( b). After amplification and filtering with a band-pass optical filter (BPF) of 3 nm bandwidth 44, a two-stage fiber interferometer circuit of 27-1 bit delay-length optically multiplexed (MUX) the signal to 160 Gb/s bit-rate 50. Its output pulse FWHM was 1.6 ps. FIG. 3( c) shows the 160 Gb/s eye diagram measured using a 65 GHz PD on an 80 GHz sampling scope.

The DEMUX setup includes a polarization controller (PC) 52 to optimize the launch polarization of the 160 Gb/s signal and an optical delay line (ΔT) 54 to align the pulse with respect to the modulator driving voltage, which was kept at the same amplitude as described above for the CW measurements.

In place of 40 GHz clock recovery from the 160 Gb/s signal, the drive voltage V_(P) was derived from the transmitter. The output of the FL-MZM with V_(P)˜V_(π), produced the 40 Gb/s eye output shown in FIG. 3( d).

The FL-MZM used a commercial 40 Gb/s LiNbO₃ MZM 62 (X-cut, zero chirp, and single drive) device with insertion loss of 5.3 dB, and low frequency V_(π) of 2.1 V. It was modulated by a 40 GHz sinusoid source amplified to ˜24 dBm (10 V peak-to-peak) for V_(P)˜V_(π). The FL-MZM output was characterized for continuous wave (CW) input at 1550 nm wavelength. FIG. 3( a) shows the autocorrelation of the 40 GHz pulse train generated, whose de-convolved FWHM is 5.5 ps (22% duty-cycle) assuming Gaussian shape.

The bit-error rate (BER) performance was evaluated by connecting the DEMUX output 66 to an EDFA 68, followed by 5 nm BPF 70, variable optical attenuator (VOA) 72 and 40 Gb/s PD receiver 74 before the BER tester (BERT) 76.

The BER curves in FIG. 4 are plotted as a function of received power to the PD and compared against the direct “back-to-back” (B2B) result obtained by connecting the 40 Gb/s signal from transmitter MZM directly to the VOA. For the FL-MZM, the 160 Gb/s DEMUX power penalty from 40 Gb/s B2B is ˜1 dB for all four 40 Gb/s channels at BER of 10-9, thanks largely to the low loss and low duty-cycle gate obtained. The DEMUX scheme is not limited to conventional RZ signals, and would function equally for advanced formats such as RZ-DPSK. The FL-MZM has future potential to be fabricated with a MZM and additional coupler integrated on the same waveguide to improve device compactness.

In conclusion the invention is able to provide a simple, yet highly effective optical switch using a commercial 40 Gb/s LiNbO3 MZM connected in a fiber-loop demonstrated generation of a 5.3-5.9 ps optical gate at 40 GHz repetition tunable from 1535 to 1565 nm wavelength, enabling optical time-division DEMUX of a 160 Gb/s signal with a low bit-error rate power penalty <1 dB for all four 40 Gb/s channels.

REFERENCES

[1] R. S. Tucker, G. Eisenstein, and S. K. Korotky, “Optical time division multiplexing for very high bit-rate transmission,” IEEE Photon. Technol. Lett., vol. 6, pp. 1734-1749, 1988.

[2] A. H. Gnauck, G. Raybon, Member, IEEE, P. G. Bemasconi, J. Leuthold, Member, C. R. Doerr, and L. W. Stulzl “1-Tb/s (6 170.6 Gb/s) Transmission Over 2000-km NZDF Using OTDM and RZ-DPSK Format”, IEEE Photon. Technol. Lett., vol. 15, no. 11, pp. 1618-1620, 2003.

[3] H. Murai, M. Kagawa, H. Tsuji, and K. Fuji, “EA Modulator-Based Optical Multiplexing/Demultiplexing Techniques for 160 Gbit/s OTDM Signal Transmission”, IEICE Trans. Electron., vol. E88-C, no. 3, pp. 309-318, 2005.

[4] H.-F. Chou, J. E. Bowers, and D. J. Blumenthal, “Compact 160-Gb/s Add-Drop Multiplexer With a 40-Gb/s Base Rate Using Electroabsorption Modulators”, IEEE Photon. Technol. Lett., vol. 16, no. 6, pp. 1564-1566, 2004.

[5] C. Schubert, J. Berger, S. Diez, H.J. Ehrke, R. Ludwig, U. Feiste, C. Schmidt, H. G. Weber, G. Toptchiyski, S Randel, and K. Petermann, “Comparison of Interferometric All-Optical Switches for Demultiplexing Applications in High-Speed OTDM Systems”, IEEE J. Lightwave Technol., vol. 20, no. 4, pp. 618-624, 2002.

[6] R. Salem, A. S. Lenihan, G. M. Carter, and T. E. Murphy, “160-Gb/s Polarization-Independent Optical Demultiplexing in 2-m Nonlinear Fiber”, IEEE Photon. Technol. Lett., vo. 18, no. 21, pp. 2245-2247, 2006.

[7] H. Sotobayashi, C. Sawaguchi, Y. Koyamada, and W. Chujo, “Ultrafast walk-off-free nonlinear optical loop mirror by a simplified configuration for 320-Gbit/s time-division multiplexing signal demultiplexing ,” Opt. Lett. vol. 27, pp. 1555-1557, 2002.

[8] K. Igarashi, K. Katoh, and K. Kikuchi, “Optoelectronic time-division demultiplexing of 160-Gbit/s optical signal based on phase modulation and spectral filtering”, Optics Exp., vol. 15, no, 3, pp. 845-851, 2006.

[9] M. L. Dennis, W. I. Kaechele, W. K. Bums, T. F. Carruthers, and I. N. Duling, III, “Photonic Serial-Parallel Conversion of High-Speed OTDM Data”, IEEE Photon. Technol. Lett., vol. 12, no. 11, pp. 1561-1563, 2000.

[10] M. Jinno, “Ultrafast Time-Division Demultiplexer Based on Electrooptic On/Off Gates”, IEEE J. Lightwave Technol., vol. 10, no. 10, pp. 1458-1465, 1992.

[11] E. J. M. Verdurmen, Y. Zhao, E. Tangdiongga, J. P. Turkiewicz, G. D. Khoe, and H. de Waardt, “Error-free all-optical add-drop multiplexing using HNLF in a NOLM at 160 Gbit/s”, Electron. Lett. 41, 349-350 (2005).

[12] Chou, J. E. Bowers, and D. J. Blumenthal, “Compact 160-Gb/s Add-Drop Multiplexer With a 40-Gb/s Base Rate Using Electroabsorption Modulators”, IEEE Photon. Technol. Lett. 16, 1564-1566 (2004).

[13] M. Jinno, “Ultrafast Time-Division Demultiplexer Based on Electrooptic On/Off Gates”, IEEE J. Lightwave Technol. 10, 1458-1465 (1992).

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Although the invention has been described with reference to a particular example, it should be appreciated that it could be exemplified in many other forms and in combination with other features not mentioned above. 

1. A high frequency optical processor, comprising a Mach-Zehnder amplitude modulator (MZM) embedded in a fibre mirror loop in which the loop input and output are joined by a polarization maintaining coupler.
 2. The processor according to claim 1, wherein the MZM has a modulation port to receive a modulating voltage, and a bias port to receive a bias voltage.
 3. The processor according to claim 1, wherein the MZM is modulated with sinewave voltage and bias offset so that the optical output from the fiber-loop for continuous wave (unmodulated) optical input is a train of short pulses of low duty cycle.
 4. The processor according to claim 3, wherein the optical output short pulses of low duty cycle have a repetition rate equal to the modulation frequency applied to the MZM.
 5. The processor according to claim 1, wherein the MZM is modulated with sinewave voltage and bias offset so that the optical output transmitted from the fiber loop for optical signal input is a time-division demultiplexed signal at quarter the input signal bit-rate.
 6. The processor according to claim 5, wherein the optical output has a bit rate at a quarter of the input signal bit-rate when the modulation frequency applied to the MZM is set to a quarter of the signal bit-rate.
 7. The processor according to claim 1, wherein the MZM is modulated with sinewave voltage and bias offset so that the optical output reflected from the fiber-loop for optical data signal input is the input signal with a channel removed (dropped).
 8. The processor according to claim 7, wherein the optical output is produced when the modulation frequency applied to the MZM is set to a quarter of the signal bit-rate.
 9. A method for using the high frequency optical processor according to claim 1, comprising the steps of: modulating the MZM with sinewave voltage; and providing bias offset so that the optical output from the fiber-loop for continuous wave (unmodulated) optical input is a train of short pulses of low duty cycle.
 10. A method for using the high frequency optical processor according to claim 1, comprising the steps of: modulating the MZM with sinewave voltage; and providing bias offset so that the optical output transmitted from the fiber loop for optical data signal input having a bit rate is a time-division demultiplexed signal at quarter the input signal bit-rate.
 11. A method for using the high frequency optical processor according to claim 1, comprising the steps of: modulating the MZM with sinewave voltage; and providing bias offset so that the optical output reflected from the fiber-loop for optical data signal input is the input signal with a channel removed (dropped). 